The present invention relates to a system for processing electrical signals and, more particularly, to a system for detecting the transitions between binary level signals of the type produced by a bar code scanner.
Electrical signals having transitions between binary levels are generated by many different devices, including electro-optical scanners of the type which scan bar code symbols and produce a corresponding video digital signal. Bar code symbols, such as those used in the Universal Product Code (UPC), are generally characterized as a series of parallel light and dark rectangular areas of different widths, consisting of dark bars and light intervening spaces. The various symbols are made up of different combinations and orderings of these light and dark rectangular areas.
Electro-optical scanning apparatus scans these bar code symbols on labels by detecting light which is reflected therefrom. The reflected light is incident upon a photo-multiplier tube, or other optical detector, which generates an electrical signal having one level for dark bars and a second level for light spaces. Positive-going transitions and negative-going transitions occur in the electrical signal to signify transitions between the light and dark rectangular areas of the scanned symbols. The transitions in the electrical signals are not perfectly sharp but, rather, occur over finite periods of time, thus making it difficult to determine the exact instant at which these transitions occur. Additionally, the labels may become slightly discolored for a variety of reasons, or may be printed on a colored background, or may be poorly printed with inadvertent spots and voids, thus introducing unexpected variations in the electrical signal produced during scanning. As a consequence, it is necessary to process the electrical signals in order to assess accurately the timing of these transitions and the corresponding widths of the light and dark rectangular areas making up the symbols.
Various circuits are known for carrying out such signal processing. These signal processing circuits typically include filters for removing unwanted components, and may also include threshold circuits in an attempt to reject spurious transitions. One prior art signal processing circuit is disclosed and claimed in U.S. Pat. No. 4,000,397, issued Dec. 28, 1976, to Hebert et al, and assigned to the assignee of the present invention. The Hebert et al circuit detects the zero crossings of the second derivative of the binary signal at selected gating times. These selected gating times occur whenever the first derivative of the binary signal exceeds a threshold level. The threshold level may be determined by adding the peak-to-peak levels of the binary signal to a rectified noise measurement signal.
Although many such signal processing circuits have been used successfully, a need continues to exist for an improved signal processing system which allows for greater precision in the detection of the transitions of a binary signal.